The present invention relates to a cage for use in a rolling bearing and, more in particular, it relates to a cage for use in a rolling bearing used, for example, in general industrial machines, machine tools and machines for iron and steel making.
Heretofore, soft steel and high strength brass have been used as metal materials for cages for use in rolling bearings. The high strength brass has self lubricity but since it requires high material cost, needs high working cost since fabricated by machining and provides low material yield, it is restricted to special application uses. With the reasons described above, SPCC materials have mostly been used as the metal material for the cage for use in rolling bearings, and cages for use in rolling bearings have been manufactured from steel plates by press molding while taking advantage of their molding workability.
SPCC materials have high general purpose applicability and is manufactured into various parts by press molding and the compositional range thereof specified according to JIS standards is extensively wide as C content of 0.12% or less and Mn content of 0.5% or less.
Cages for use in the rolling bearing manufactured by press molding from deep drawn steel plates as the SPCC materials with extremely reduced C content of as low as the order of 0.001% have low yield strength because of extremely low C content, so that the cage for use in the rolling bearing may possibly lack in the strength. That is, since the levels for the strength of the SPCC materials differ even when the composition is within the range of JIS standards, the strength of the cage varies sometimes failing to obtain stable performance.
Further, even for an identical composition, since the hardness changes depending on the difference of the state of annealing, the hardness, that is, the strength may possibly vary greatly to result in a problem that a stable performance as the cage for use in the rolling bearing can not be obtained easily.
On the other hand, working conditions for rolling bearing have become more stringent in recent years and high load capacity rolling bearings have been developed in recent years. For coping with the high load capacity, a method of decreasing the diameter of a roller to increase the number of the rollers has been adopted or the shape of the cage for use in the rolling bearing has been optimally designed, which makes the shape complicated.
Particularly, in the case of the method of increasing the number of rollers by increasing the load capacity, since the space between the rollers is decreased, it is necessary to make the bar of the cage for use in the rolling bearing finer. Accordingly, depending on the composition and the hardness of the SPCC material used, it may be a worry that the strength of the bar of the cage for use in the rolling bearing becomes insufficient to cause deformation, or plastic deformation is caused at the surface of contact with a rolling element of a rolling bearing, so that the shape and the dimensional accuracy of the cage for use in the bearing can no more be maintained to possibly cause fracture in the worst case.
In view of the above, it is a subject of the present invention to overcome the problems in the existent cages for use in a rolling bearing as described above and provide a high strength cage for use in a rolling bearing less causing deformation or fracture and having stable performance.
For solving the foregoing subject, the present invention comprises the following constitution. That is, a cage for use in a rolling hearing in accordance with the present invention provides a cage made of a low carbon steel used in a rolling bearing, comprising an outer ring, an inner ring, a plurality of rolling elements disposed rotationally between the outer ring and the inner ring and having pockets for retaining the rolling elements in an equi-distribution in the rotating direction of the rolling elements, wherein the hardness for the surface at least at a portion of contact with the rolling element is Hv 190 or more and the hardness at the inside or core portion of the cage is Hv 110 to 170.
With such a constitution, the cage for use in the rolling bearing can be kept at a high strength and deformation or fracture at a portion of contact with the rolling element of the rolling bearing that generates impact load can be suppressed.
When the hardness for the surface of the cage for use in the rolling bearing at least for a portion of contact with the rolling element is less than Hv 190, plastic deformation tends to be caused by impact load by the contact with the rolling element of the rolling bearing tending to cause deformation or fracture to the cage.
For suppressing the disadvantage described above, the hardness for the surface of the cage for use in the rolling bearing at least for a portion of contact with the rolling element is preferably Hv 200 or more.
Further, if the hardness for the inside of the cage for use in the rolling bearing is less than Hv 110, the cage tends to cause deformation or fracture and, on the other hand, when it exceeds Hv 170, although the strength of the cage is improved, the moldability is lowered to cause difficulty in obtaining a predetermined dimensional accuracy.
For further suppressing the deformation or fracture, the hardness for the inside of the case for use in the rolling bearing is preferably Hv 120 to 170. Further, considering the moldability upon press molding or the like, Hv 110 to 160 is preferred.
Further, the ratio between the yield strength and the tensile strength (yield strength/tensile strength) of the low carbon steel is preferably from 0.65 to 0.75. When the ratio between the yield strength and the tensile strength of the low carbon steel is less than 0.65, the cage lacks in the strength and it is difficult to obtain a predetermined dimensional accuracy. For suppressing such a disadvantage, the ratio between the yield strength and the tensile strength of the low carbon steel is more preferably 0.70 or more. A higher ratio for the yield strength and the tensile strength of the low carbon steel is desirable but 0.75 is a limit value (upper limit) with a commercial point of view (refer to FIG. 1).
Further, the low carbon steel may be a steel containing from 0.02 to 0.10% by weight of C and from 0.10 to 0.45% by weight of Mn.
C has an effect of solid solubilized in the matrix ferrite to remarkably increase the strength and, since this effect is fully attained by 0.02% by weight or more, the C content is preferably 0.02% by weight or more. However, when it is contained in excess of 0.10% by weight, the effect is no more improved but saturated. Further, the ductility is lowered to deteriorate the moldability and it is difficult to ensure the dimensional accuracy of the cage for use in the rolling bearing after pressing in the case of press molding, so that the C content is preferably 0.10% by weight or less. Accordingly, the C content is preferably from 0.02 to 0.10% by weight.
Further, Mn has an effect of solid solubilized in the matrix ferrite to strengthen the material and, since this effect is fully attained by 0.10% by weight or more, the Mn content is preferably 0.10% by weight or more. On the other hand, when Mn is added in excess, the moldability is deteriorated and cracking tends to occur during molding in the case of the press molding, so that it is preferably 0.45% by weight or less. Accordingly, the Mn content is preferably from 0.10 to 0.45% by weight.